1. Field of the Invention
The present invention relates to producing silver films, and more particularly, to films having large particles caused by cracking during anaerobic annealing to provide surfaces that exhibit metal enhanced fluorescence.
2. Related Art
In recent years it has been both described and demonstrated many new favourable photophysical effects of electronically excited states (fluorophores/luminophores) in close-proximity to plasmon resonant particles have been both described and demonstrated. (1-3) The coupled fluorophore lifetime is also observed to be much shorter than the “free-space lifetime”, reflecting the very fast “plasmon” lifetime of the coupled quanta. (5-7) Further, it is thought that the MEF effect is a consequence of an excited state coupling with the scattering mode of nanoparticles, which is thought to account for very fast MEF lifetimes, i.e. a coupled elastic scattering event. (3,8) For metallic nanoparticles, the extinction spectrum is comprised of both an absorption and scattering component. (9) For smaller particles (<25 nm), then the extinction is for the most part dominated by absorption, while for larger particles, by scattering. For a fixed wavelength, this scales as the radius cubed and to the sixth power respectively. (9) Given that MEF is thought to be the ability of a nanoparticle to plasmon scatter coupled quanta (i.e. fluorescence, phosphorescence etc), (3) then concordantly larger particles would certainly be ideal for MEF. (3,8)
It is this train of thought which readily accounts for the numerous reports of luminescence quenching by small metallic nanoparticles. While the mechanism for MEF is fairly new, (3) compelling data has been reported wherein: i) MEF is seen to be more pronounced for larger particles; ii) the wavelength dependence of MEF suggests a correlation between the scattering spectra of the nanoparticles and the emission spectra of the fluorophores; iii) MEF is angular dependent, (10) both from an observation and excitation perspective, similar to scattering by nanoparticles themselves (11); iv) metals ideal for MEF are those with high free electron densities (12); and v) continuous metallic films have been shown to couple fluorophore emission, when fluorophores are positioned less than 100 nm from the surface. In addition, the coupled emission has been shown to be completely p-polarized, strongly indicating that the coupled-plasmon system is radiating. (13,14)
Silver is usually the noble metal of choice (12, 15-17) for applications in MEF and Surface-Enhanced Raman Spectroscopy. The preparation of silvered substrates include several modes of deposition, such as by wet chemistry, (18) a layer-by-layer deposition technique, (19) electrochemically, (20) on glass, (21) plastic substrates, (22) and on indium tin oxide. (23) One of the most commonly used techniques is vapor-deposition (21, 24-27) as it yields the most reproducible substrates amongst the deposition techniques aforementioned. After the deposition of silver films onto substrates, a high-temperature post-annealing process can also be applied to introduce a quasiperiodic roughness that further increases the utility of these surfaces. (21,28,29)
However, heretofore producing larger nanoparticles with consistency in size has been difficult to grow/produce by other wet-chemical deposition techniques without forming a continuous film. Thus, it would be advantageous to develop a method of forming large particles that do not spread into a continuous film.